The Underground layer is a level where there is a transition between dirt background that can be removed and dirt/rock background that cannot be removed (note that walls can be placed on this irremovable background).

Sky, ground level, stone layer and underground layer (underground jungle being part of it) seem to be affected from scaling while the size of the dungeon (not including the corridor leading down) and Underworld (always 180-200 blocks from the lower border of the world) seem to be affected only slightly. Depth of the Corruption chasms are also affected only slightly with a small chance of spawning chasms down to the underground level, regardless of how deep they are.

It appears that, in larger worlds, resources are much farther apart from each other in comparison to the smaller world types. This means that in smaller worlds, resources are much easier to locate and mine - see the map viewer fragments in the gallery below for comparison. On a large map, corruption can, although rare, stretch down far underground, potentially reaching close, or to, the Underworld.

Shots were taken from the zone 600x450 blocks at corrupted area for better comparison.

Bugs

In any lighting mode other than Trippy or Retro (color and white), the 17 blocks at either end of the world will be cut off by the screen and the player cannot proceed past that point. This can be annoying as sometimes there will be ocean/shadow chests hidden off screen. One can quickly fix this by pressing F9 to change the lighting mode to Trippy or Retro, and then changing back after you have explored the area.(fixed as of 1.2)